Axial trajectory sensor having gating means controlled by pulse duration measuring for electronic particle study apparatus and method

ABSTRACT

A sensor for use with apparatus operating in accordance with the principles of the Coulter electronic particle studying device, for discriminating between signals from particles passing on axial or near axial paths through an aperture and particles passing off center on the basis of their differing durations. The pulse duration of a portion of the pulse is measured and only those which meet the criteria of duration established by the electronic circuitry are permitted to pass for use in pulse height analysis apparatus following the sensor. The other pulses are discarded on the basis of their greater durations. The apparatus provides structure for deriving a duration-measuring pulse whose duration extends in one case from the peak of a particle pulse to a fractional amplitude thereof, and in another case from the time of maximum slope of the leading edge to the time of maximum slope of the trailing edge. In each case the duration-measuring pulse is converted into a signal which has an amplitude proportional to duration and the latter signal is compared with a certain maximum signal level to operate gating means for rejecting the longer duration pulses and passing the shorter duration ones. Multiple peak pulses are also discarded by means of suitable circuitry.

Jan. 1, 1974 AXIAL TRAJECTORY SENSOR HAVING GATING MEANS CONTROLLED BYPULSE DURATION MEASURING FOR ELECTRONIC PARTICLE STUDY APPARATUS ANDMETHOD Inventors: Walter R. I-Iogg; Wallace 11.

Coulter, both of Miami Springs, Fla.

Assignee: Coulter Electronics, Inc., Hialeah,

Fla.

Filed: Feb. 8, 1971 Appl. No.: 113,165

Primary Examiner-John W. l-luckert Assistant Examiner B. P. DavisAttrneySilverman & Cass 7 ABSTRACT A sensor for use with apparatusoperating in accordance with the principles of the Coulter electronicparticle studying device, for discriminating between signals fromparticles passing on axial or near axial paths through an aperture andparticles passing off center on the basis of their differing durations.The pulse duration of a portion of the pulse is measured and only thosewhich meet the criteria of duration es- [52] US. Cl 328/111, 328/92,324/71 P tablished b the electronic circuitr are ermitted to [51} ltCll-l03k520 y p 8 Fl} pass fol-use m pulse height analysis apparatus f [5Ield of Search 307/234, 267; i the Sensor The other ulses are discardedon the 328/111 117 146 149' 324 71 PC p 1 basis of their greaterdurations. The apparatus provides structure for deriving aduration-measuring pulse [56] References C'ted whose duration extends inone case from the peak of a UNITED STATES PATENTS particle pulse to afractional amplitude thereof, and in 2.996.624 8/1961 Mumma 328/116 uxanothet case from the time of maximum Slope of the 3,502,973 3/1970Coulter 235/92 PC X leading edge'to the time of maximum slope of the3.548206 12/1970 Ogle et a1. 326/116X trailing edge. In each case theduration-measuring 3.553.593 1/l97l Gedance 1 1 328/112 pulse isconverted into a signal which has an ampli- H1971 88 cl 1- 235/1513 tudeproportional to duration and the latter signal is 3222 compared with acertain maximum signal level to op- 2,968.01! 1 1961 Schouten et al...333/ i g i f j g h longer fi 2 99 24 19 Mumma 250 21 P 535 an P g t eOtter ration onesp 3.255.293 6/1966 walker 84/1) peak pulses are alsodiscarded by means of suitable 3,289,195 ll/1966 Townsend... 340/324circuitry- 3.399,3l| 8/1968 Andrea 307/225 3.454.792 7/1969 Horlander307/261 36 Clams 7 Draw Flgul'es W029i 3 7 44/11 I 6475 mm 340 JIM/21V,52?

l ?/5Z kaa4 a :Mzz 34/ fl/I/it J 41/4444 1'66 F zz/gr ldtf/A/f- [L Vgdni 17507 3 /p6 z. 5- 6 /Z/fl/fl r adda .GA/A/ 3 52 Z474) 40- 57 igrzl/m/a 3 0 I16! 7 2:71am 3 762 3 72 24a 3'62 1 p g. 711/2/4/4 j? a![ml/17M FWTJZZX il 34v 44.40.60 7 M1 I j 22% i 544 v [75772539922253 174K) 1 I II/lK/FZ r a I z-no aw/z H'M 5 p ,u 6 7114M o/vz PAIENTED 1 74SHEET 2 0F 6 wwsx SHEET 6 BF 6 PATENTED JAN 1 74 AXIAL TRAJECTORY SENSORHAVING GATING MEANSCONTROLLED BY PULSE DURATION MEASURING FOR ELECTRONICPARTICLE STUDY APPARATUS AND METHOD CROSS REFERENCE TO RELATEDAPPLICATIONS This application is concerned with modified forms of theinventions disclosed in co-pending applications having the same title asthis application Ser. Nos. 84,440 and 101,325, filed respectively onOct. 27, 1970 and Dec. 24, 1970, one of the applicants herein being theapplicant in both of said co-pending applications, and all of theapplications being assigned to the same assignee.

BACKGROUND OF THE INVENTION scopic particle in suspension in anelectrolyte is passed I through an electrical field of small dimensionsapproaching those of the particle, there will be a momentary change inthe electric impedance of the electrolyte in the ambit of the field.This change of impedance diverts some of the excitation energy into theassociated circuitry, giving rise to an electrical signal. Such signalhas been accepted as a reasonably accurate indication of the particlevolume for most biological and industrial purposes. Apparatusembodyingthe teachings of U.S. Pat. No. 2,656,508 has been used to count and sizeparticles in biological fluids, industrial powders and slurries, etc.

The principles of the present invention apply to Coulter particleanalyzing apparatus in which the excitation of the field is achieved bymeans of unidirectional or low frequency power sources or radiofrequency power sources.

ln commercial versions of the Coulter particle analyzing apparatus, theelectric field of small dimensions has been formed commonly by amicroscopic right cylindrical passageway or aperture, as it is known,between two bodies of liquid in which the particles to be studied aresuspended. The electrical excitation energy is coupled to these bodiesby means of electrodes respectively located in the liquid bodies, theaperture being formed in an insulating wall between the bodies. Thesuspension is caused to flow through the aperture carrying the particleswith the flow and giving rise to the electric signals produced by themomentary changes in impedance caused by the respective particles asthey pass through the aperture. The electric field is concentrated inthe aperture and normally comprises an electric current flowing throughthe aperture along with the physical flow of suspension.

By counting the signals produced, one can count the particlespassing'through the aperture. By discriminating between different pulseamplitudes, one can make size studies. This invention is primarilyconcerned with size studies, and has, as a very important objectthereof,

the provision of apparatus which will enable highly accurate particlesize data to be achieved.

It has been known that long apertures can produce results which aresuperior to short apertures insofar as size measurements are concerned,if the bandwidths of the associated amplifiers are reduced accordingly.A long aperture may be considered one in which the length is greaterthan the diameter. The usual Coulter aperture is relatively short, thatis, its length is the same as or less than its diameter.

The reason for better size information with long apertures is that theelectrical field halfway through the aperture, being the position mostremote from the entrance and exit of the aperture, is most uniform andhas the most uniform current distribution for all paths through theaperture. The longer the aperture, the more nearly uniform is the fieldat this midpoint. At the entrance and exit of the aperture, the currentdensity is greater at the edges of the aperture and correspondinglylesser on the axis of the aperture. This may be explained by pointingout that current paths other than the axial path are supplied from thesides of the aperture as well as straight ahead. The lower currentdensity on the axis at the entrance and exit results in a lowerinstantaneous signal than is the case for particles entering theaperture and leaving it on other paths. In other words, the currentdensity at the corners of the aperture is greater than at the axis.

Another phenomenon is important to consider, according to thisinvention. The velocity of electrolyte flow, and hence the velocity ofparticles also, is somewhat greater on an axial path than on pathscloser to the edges of the aperture or paths which are off-center,because the liquid does not have to change direction when it goesthrough the axial center of the aperture. The resistance to flow is aminimum on the axis since it is surrounded by a moving sheath of liquidhaving substantially the same velocity.

The prior art has recognized the problem involved in the use of theCoulter apparatus for sizing studies, but so far as is known, there hasbeen no satisfactory solution. One attempt involved releasing theparticles in a suspension from a focussed source ahead of the aperture;but this involved the use of two apertures and the inability toilluminate and view the aperture during the process.-

The use of long apertures poses too many problems to make the samepractical. The long aperture has less sensitivity. lt adds resistance tothe effective aperture which generates noise tending to mask thesignals. Mi-

crophonic modulation of the aperture is also increased.

the said co-pending applications. The invention of this applicationrelates to forms of the basic invention which are directed to economicsin circuitry by eliminating some of the more expensive components usedin the circuits of the co-pending applications.

The invention herein also attacks the problem of processing pulses whoseleading and trailing edges are not basic invention which is disclosed invarious forms in well-defined and discriminating against pulses whichhave multiple peaks. In the latter case, the multiple peak signifies apulse produced by a particle which passed through two locations of highcurrent density while traversing the aperture. Its amplitude isanomolous and hence not proportional to the size of the particle.

SUMMARY OF THE INVENTION According to the invention, particles passingthrough an aperture are examined electronically to ascertain which ofthem passed most nearly on axial paths through the aperture. These arethe only particles which are permitted to be regarded by the apparatus,the others being disregarded. The electronic selection is based on thefact that the particles following axial paths spend the least time inthe ambit of the aperture, and therefore their corresponding pulses havethe shortest duration. Ideally, all pulses passing through the aperture,regardless of size, will have the same duration; but because of thereasons given above, this is not practically true. Pulses due toparticles which pass through the aperture off-center will normally havelonger durations.

By disregarding a percentage of the pulses, fewer are considered by thepulse height analyzing equipment which follows the sensor of theinvention, resulting in a slight degradation in the statistical accuracyif a given amount of sample of a given concentration is scanned. Thedata which are achieved, however, are of much higher quality. If a countis required, this is made before the signals are processed in the sensorof the invention.

The particle pulses are examined by ascertaining their durations fromthe first peak of the pulse to some fraction of its amplitude. The peakis found by differentiating the particle pulse once and choosing thepoint at which the resulting signal passes through zero. This signifieszero slope or the peak of the particle pulse. The resulting measuringsignal is then converted into a pulse whose amplitude is proportional tothe duration of the measuring signal. This amplitude is then comparedwith certain signal level criteria to ascertain whether the originalparticle pulse was of a size to be passed to the pulse height analyzingequipment or to be disregarded.

In another version of the invention the pulses are differentiated twiceresulting in a wave or signal representing rate of change of slope. Atmaximum slope the rate of change of slope is zero and hence the signalcrosses the base line for each point of inflection of the leading andtrailing edges of the particle pulse, i.e., where the pulse shapechanges from concave to convex, and it is processed in the same manneras the duration-measuring pulse described above. Additionally, means areprovided to reject any pulses which produce more than two zero crossingsof the base line since this will clearly result when the particle pulsehas more than one peak.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a diagrammatic view of theaperture of a Coulter particle analyzing apparatus showing the paths ofdifferent particles through the apparatus;

FIG. 2 is a diagram showing the graphs of particle pulses resulting fromthe passage of the particles of FIG. 1 along the paths shown through theaperture;

FIG. 3A is a block diagram of an axial trajectory sensor constructed inaccordance with the invention;

FIG. 3B is a diagram consisting of a series of graphs all on the sametime scale illustrating various wave shapes throughout the sensor ofFIG. 3A resulting from the processing of two particle pulses therein;

FIG. 4A is a block diagram of an axial trajectory sensor of a modifiedform;

FIG. 4B is a diagram consisting ofa series of graphs all on the sametime scale illustrating various wave shapes throughout the sensor ofFIG. 4A resulting from the processing of two particle pulses therein;and

FIG. 5 is a block diagram of a system constructed in accordance with theinvention and using an axial trajectory sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The basic concept of thisinvention is common to the two co-pending applications and like them isbased upon a practical consideration of the electric pulses which resultwhen a suspension of particles is passed through the aperture ofv aCoulter electronic particle analyzing device. As explained in theco-pending applications, particles which pass through the aperturesubstantially along the axis thereof will produce electric pulses whosedurations are quite closely the duration of the particle within thepractical ambit of the aperture, and will produce electric pulses whoseamplitudes are quite closely proportional to the sizes of the respectiveparticles producing the same.

Since the practical aperture of the Coulter device is formed in a waferof some hard material such as corundum, the techniques of manufactureresult in apertures with generally sharp entrances and exits. Suchapertures are also more desirable than those which have roundedentrances and exits since they do not as readily clog with debris, andif clogged are more easily cleared than the rounded entrance ones.

Particles which pass through the ordinary Coulter aperture off centerwill produce electric signals which have durations longer than thosewhich pass through the aperture on center. They may additionally havemultiple peaks and amplitudes which are either anomalous or notproportional to the respective sizes-of the particles which cause thesame.

The reasons for the phenomena above-mentioned are stated in some detailin the said co-pending applications but will be discussed very brieflyhere. The speed of the flow in the center of the aperture is greaterthan off center. Along the walls of the aperture with sharp entrancesand exits there is a slower flow which slows down the passage ofparticles and hence their durations of passage as well as the durationsof their electric pulses will be greater than those of particles passingthrough the center of the aperture. Additionally, there are locations ofhigh current density in the vicinities of the corners which may producepeaks in the resulting particle pulses. Normal electronic circuitry isnot capable of ascertaining without error that instantaneous value of amultiple peak pulse which truly represents the amplitude that isproportional to size.

Like the inventions of the co-pending application, the function of theinvention herein is to accept pulses of the shorter durations anddiscard pulses of the longer duration. The object of improving thequality of the pulses is to make analyses of the particle sizes; hence,

if any counting is to be done, this is accomplished prior to theintroduction of the particle pulses to the sensor of the invention.

The sensor of thisinvention differs from those of the co-pendingapplications'in that circuitry is used which eliminates the mostexpensivecomponents of the oth ers. Additionally, circuitry is disclosedwhich discards any pulses which have more than one peak, irrespective oftheir duration.

A system constructed in accordance with the invention is illustrated inFIG. 5. The block comprises a Coulter particle analyzing apparatus whichis normally composed of a stand, detector and counter. The standincludes the vessels, aperture tube, fluid system. and electrodes of theapparatus. The detector includescir cuitry which produces the particlepulses. The counter may be any device which responds to the particlepulses, and may include pulse: height discriminating means. It may beomitted in instances where only size studies are to be made, but isshown in order to point out that since the sensor 3-10 will bediscarding many pulses, it is best to make any counts prior to applyingthe particle pulses to the sensor 3-l0. As seen, from the sensor 340,the output signals at 3-78 are applied to some form of pulse heightanalyzer 14 in order to make the sizing studies.

FIG. 1 is a diagrammatic view of an aperture which constitutes thescanning means in the stand of Coulter electronic particle device 10,immersed in a liquid and having particles passing through the apertureof the wafer. Thus', the wafer is designated 20, and the aperture itselfis designated 22. The sample liquid is passing through the aperture 22from right to left, and as it moves, it carries theparticles insuspension with it. The paths of three particles, X, Y, and Z, areillustrated at 24, 26, and 28, respectively. These paths aredeliberately chosen to be considerably different, for purposes ofillustration, and the-signal or particle pulses which are produced as aresult of such passage are shownon the identical time base in FIG. 2 atgraphs X, Y, and Z.

The particle X passes almost coaxially of the aperture 22 along the path24. The speed of the liquid passing through'the aperture at this pointis maximum and the current density distribution along the path is mostpredictable. Accordingly, the resulting pulse 30 in FIG. 2, as shown inthe curve X is a simple bell-shaped pulse whose duration is proportionalto the length of the aperture 22 from t, to and whose amplitude is quiteclosely proportional to the size of the particle. Although the amplitudewill be considered as voltage, it should be understood that pulses andsignals could also be current waves.

The particle Y passes through the aperture 22 on a diagonal path 26. Inthe first place, it will be appreciated that its path, while traversingthe aperture, is longer than the path 26 because it is at an angle. Inthe second place, at the point where it entered the aperture, this beinga corner at 32, the current density is much higher than that closer tothe axis of the aperture. Accordingly, the beginning of the pulse34'which is produced by this particle, will have a higher amplitude, andwill also probably commence slightly before the pulse 34. If itcommences at practically the same time t,, due to its time within theaperture being longer, it will finish later than the time As shown,there is a peak at 36 due to the effect of high current density at thecorner 32, and a lesser peak at 38 which is produced when the particleleaves the aperture, since it is approaching the high current density atthe corner 40.

The particle Z goes through the aperture 22 on a relatively straightline, but in this case it is quite close to the wall 42 of the aperture.The resulting pulse 42' has two peaks, one at 44 caused by the corner 32with its high current density, and the other at 46 caused by theparticle passing the high current density corner 48. In this case, theparticle will remain in the aperture longer than the time t, to becausethe speed of flowing liquid is less adjacent the wall than it is in thecenter of the stream. This is a well-known phenomenon of flow of liquidsthrough orifices.

In these three cases, it can be seen that the only pulse which is-mosttruly representative of the size of the particle is that which passesthrough the center of the aperture 22, namely, the particle X. Accordingto the invention, circuitry is provided to discard pulses of the othertypes, based upon their time duration, since it becomes clear that onlythe pulses of shorter duration have gone through the center of theaperture and produced the most representative wave shapes.

According to the invention, structure is provided to discriminatebetween different types of pulses which are illustrated in the graphs ofFIG. 2. The basis for duration discrimination in the apparatus which isdescribed in connection with the circuits detailed herein is analog innature, although as explained in the first of the co-pendingapplications, such basis could as well be digital. The structure whichis described hereinafter for use in duration discrimination issubstantially identical to that used in the second of the co-pendingapplications, and obviously other means could be used. The structurewhich is described hereinafter to discriminate against particle pulseswhich have more than one peak is somewhat digital in nature and is usedalong with a gating signal based also upon duration discrimination,although it could be used alone under certain circumstances.

The block diagram of FIG. 3A illustrates an axial trajectory sensorwhich utilizes the basic concept of the invention and those of theco-pending applications and, in addition, is constructed in accordancewith the invention to achieve the purpose alluded to above. lts primaryattributes are simplicity and economy. For the purposes of theexplanation, it is assumed that two particles are being examined, andtheir particle pulses are being processed in the sensor of FIG. 3A.These are the particles X and Y of FIGS. 1 and 2 and the resulting waveshapes derived from the pulses throughout the circuit are illustrated inFIG. 38, all on the same time base.

The apparatus of FIG. 3A is designated generally 3-10 and ischaracterized by the provision of means to discriminate between pulsesof different durations, on the basis of which the desired pulses arechosen and permitted to pass through the apparatus in the form ofrectangular signals, the amplitudes being the same as the respectiveparticle pulses causing the same, and all durations being the same. Theparticular discriminating means used in the apparatus of FIG. 3A and aswell in the apparatus of FIG. 4A is substantially disclosed in thesecond of the co-pending applications. It is automatic in that thepulses themselves control the level of voltage representing a maximumduration against which the durations of the respective particle pulsesare measured. Additionally, the apparatus 3-10 produces itsduration-measuring signal from the time that a particle produced pulsereaches its first peak to the time that it subsides to a predeterminedfraction of its amplitude, chosen to be 50 percent in the particularexample.

The incoming particle pulses such as 3-30 and 3-34, shown in graph A ofFIG. 3B, are applied to the input terminal 3-12 and from this point areapplied to the pulse stretcher 3-26 and by way of the line 3-14 to thedifferentiator 3-16, the comparator 3-28 of the low threshold circuit3-18 and to the comparator 3-46.

The differentiator 3-16 differentiates the signal 3-30 and produces atits output 3-50 a signal 3-58 shown in the graph B of FIG. 3B. Initiallythe progress of the particle X will be examined as its pulse 3-30 isprocessed by the sensor 3-10.

The low threshold circuit 3-18 comprises the voltage reference source3-22, the potentiometer 3-24 and the comparator 3-28. The low levelthreshold voltage is chosen to suppress noise, is slightly above thebase line of the incoming signals and is shown at 3-32 of graph A ofFIG. 3B. When the signal 3-30 arrives, at the time 2., close to thebeginning of the pulse, it exceeds the low threshold voltage 3-32 and anoutput is produced on the line 3-36 from the comparator 3-28 whichcomprises the rectangular wave 3-38 of graph C of FIG. 3B. The wave 3-38persists so long as the signal 3-30 exceeds the low threshold 3-32 andhence lasts until the time 1 The beginning of the rectangular wave 3-38occurs at the time t The leading edge of the wave 3-38 is sensed by theleading edge detector 3-42 to produce the trigger spike 3-44 at the timet. shown in the graph F of FIG. 3B. This trigger spike 3-44 is used toset the RS flip-flop 3-48. The purpose of this component will bedescribed below. 7

The output of the differentiator 3-16 comprising the signal 3-58 isapplied by way of the line 3-50 to the comparator 3-52. It will be seenthat the second terminal input of the comparator 3-52 is grounded asshown at 3-54 so that the only output from the comparator 3-52 appearingat the line 3-56 will occur while the input on the line 2-50 ispositive. The output of the comparator 3-52 therefore consists of arectangular wave 3-60 shown in graph D of FIG. 3B which is produced onlyfor that portion of the signal 3-58 which is positive.

At this point, the significance of the rectangular wave 3-60 should beexamined. Since the original particle pulse 3-30 is applied to thedifferentiator 3-16, the output signal 3-58 is a graph of the slope ofthe pulse 3-30. The slope of the leading edge of the pulse 3-30increases from the base line until it is a maximum at about the centerof the rise, after which it decreases until at the peak the slope iszero. Thus, the wave 3-58 has a peak in about the center of the rise ofthe leading edge of the pulse 3-30, then decreases to zero which is thetime that the peak of the pulse 3-30 is reached. Thereafter, the slopebecomes negative and a waveform similar to the first half of the wave3-58 is generated, but below the base line since the slope is negative.The crossing of the base line at the time r thus very accuratelyestablishes the peak of the pulse 3-30, and when the rectangular wave3-60 is generated, its trailing edge occurs at the exact time of thepeak of the pulse 3-30. Passing the signal 3-60 through a trailing edgedetector 3-62, there is derived on the line 3-64 as the input to thereset terminal of the RS flip-flop 3-48 a trigger pulse 3-66 whichestablishes also the exact time that the peak of the pulse 3-30 occurs.As stated, this is at the time t,.

The effect of the trigger spike 3-66 is to reset the RS flip-flop 3-48,it being recalled that the trigger spike 3-44 set the RS flip-flop atthe time t,. As will become apparent from the description that follows,the RS flipflop 3-48 permits the sensor 3-10 to respond only to thefirst peak of any incoming particle pulse, it being assumed that anymultiple-peaked pulses will be rejected on the basis of duration.

The output of the RS flip-flop 3-48 appears at the line 3-80 andcomprises a rectangular wave 3-74 shown at the graph G or FIG. 3B. Thispulse is applied to the trailing edge detector 3-82 whose output thenprovides the trigger spike 3-96 of graph H of FIG. 3B which, in turn, isapplied to the set input of the RS flipflop 3-100. The output of the RSflip-flop 3-100 starts at the time t which marks the exact instant thatthe peak of the pulse 3-30 is reached, and continues as a rectangularwave 3-104 of graph L of FIG. 38 until the flip-flop is reset. The resetsignal consists of a signal appearing at the reset terminal of the RSflip-flop 3-100 derived from the trailing edge detector 3-102 on theline 3-101. The rectangular wave 3-104 commences at the time t and endsat the time 1 The pulse 3-104 is used to open the analog gate 3-20 towhich the RS flip-flop 3-100 is connected by the line 3-106. When theanalog gate 3-20 is turned on, current from the constant current supply3-114 commences to flow into the integrator 3-58 by way of the line3-122. This commences the production of a ramp and plateau pulse 3-126shown in graph M of FIG. 38, with the ramp portion being produced by theflow of current and so long as the current is flowing. This means thatthe amplitude of the plateau portion will be proportional to theduration of the pulse 3-104, or proportional to the duration of timemeasured between 1 and It has already been explained that t is theinstant that the peak of the pulse 3-30 occurs, and the followingexplanation will show the derivation of the time [3.

The pulse 3-30 was also applied to the pulse stretcher 3-26, and thestretched pulse resulting is the pulse 3-128 of the graph I of FIG. 3Bappearing on the line 3-156 and at the input to the attenuator 3-40. Thepulse 3-128 has an amplitude which is the same as the pulse 3-30,namely, the amplitude X, but the circuit retains the voltage levelreached by the leading edge of the pulse 3-30 and thus produces aplateau of constant amplitude until the condensers in the circuit aredischarged. The stretched pulse 3-128 is attenuated in the attenuator3-40 to some amplitude which is a predetermined fraction of theamplitude X, chosen in this case to be 50 percent. The output of theattenuator 3-40 appears at the line 3-136 and consists of the signal3-140, shown in graph J of FIG. 3B. The flat-topped pulse 3-140 ispassed to the clipper 3-134 on the line 3-136. This prevents the voltageof the pulse 3-140 from decreasing to ground reference. Instead, itremains at the level 3-142 between pulses such as 3-140, slightly abovethe base line 3-146. The purpose of this is to ensure that betweenpulses the voltage at the path 3-148 will be below the voltage on thepath 3-152.

The output of the clipper 3-134 appears on the line 3-152 which is oneof the terminals of the comparator 3-46, the other terminal having theoriginal particle pulse 3-30 thereon. This is shown with the two waves3-140 and 3-30 superimposed in graph J of FIG. 3B. The two signals arecompared, and the comparator 3-46 produces an output of constantamplitude for the time that the pulse 3-30 exceeds the signal 3-140.This occurs from slightly after the time t, to the time t and theresulting wave 3-154 appears on the line 3-160 and is shown in the graphK of FIG. 38. From the comparator 3-46 the signal 3-154 is applied tothe trailing edge detector 3-102, this detector producing a triggerspike 3-166 shown in graph of FIG. 3B. The trigger spike 3-166 appearson the line 3-101.

Considering the processing-of the pulse 3-30 through the pulse stretcher3-26, attenuator 3-40, clipper 3-134, comparator 3-46 and the trailingedge detector 3-102, it will be'noted that the time which is that timeat which the trigger spike 3-166 appears is the instant that theparticle pulse 3-30 has subsided to the fractional amplitude chosen bythe attenuator 3-40, in this case 50 percent of its original amplitudeX. This part of the pulse is on its trailing edge.

In review, we now have two trigger pulses 3-96 and 3-166 whichrespectively occur at the instant that the pulse 3-30 reaches its peakand the instant that it subsides to 50 percent of its amplitude X. Ithas been explained that the trigger spike 3-96 sets the RS flip-flop3-100 to start the rectangular wave 3-104, and now it can be seen thatthe trigger spike 3-166 is applied to the reset terminal of theflip-flop 3-100 so that the rectangular wave 3-104 is terminated at thetime t .'The duration of the rectangular wave 3-104 fromthe time 1 tothe time is the duration of the pulse 3-30 from its peak to the halfamplitude of its trailing edge. The rectangular wave 3-104 produces theoutput 3-126 from the integrator 3-58, this being a ramp and pedestalwave 3-126 of graph M previously described. The length of the pedestalor plateau is determined by the time when the shorting means of theintegrator 3-58 is energized to reset the integrator to its minimalcharge condition. The trigger spike 3-166 is applied by the line 3-101to the one-shot multivibrator 3-108 which commences to produce arectangular pulse 3-168 at the time t This pulse' is shown at graph P ofFIG. 3B.

The rectangular pulse 3-168 is applied at 3-110 to the trailing edgedetector 3-112 and by way of the line 13-170 to the AND gate 3-130 andthe input line 3-132 of the minimum duration memory circuit 3-70. As forthe pulse applied to the trailing edge detector 3-112, this produces atrigger pulse at the end of the duration of the one-shot multivibrator3-108.'This duration is chosen to be sufficient to provide awell-defined output pulse from the sensor 3-10. In the exampledescribed, this pulse extends from the time 1 to the time i The outputpulse will always have this exact duration. At the time a trigger spike(not shown) appears at the input 3-116 of the one-shot multivibrator3-118, and this multivibrator puts out a rectangular pulse 3-172 shownin graph Q of FIG. 3B of sufficient length to discharge the pulsestretcher 3-26 by way of the line 3-124 and to short the integrator3-58. This occurs after the processing of a particle pulse 3-30 has beencompleted and renders the sensor 3-10 ready to receive the next pulse.

The AND gate 3-130 will pass the rectangular signal 3-168 to the line3-158 to open the analog gate 3-76 for the time L, to t only if there isa signal at both of its input terminals 3-164 and 31-170. Ithas beendemonstrated that the signal 3-168 will occur in every case on the line3-170. It remains to be shown how the output from the integrator 3-58appears on the line 3-68 and when compared to whatever signal appears onthe line 3-172 in the comparator3-l50 may provide a signal on the line3-164.

The signal 3-104 is a duration-measuring signal or pulse and the rampand plateau pulse 3-126 is a signal or pulse whose amplitude isproportional to the time measured by the duration-measuring pulse 3-104.The amplitude of pulse 35-126 is an electrical quantity whose value isproportional to the time duration of the duration-measuring pulse 3-104.Accordingly, as taught by the co-pending applications, we may establishsome electrical effect having a standard against which to compare theamplitude of the integrated pulse 13-126 by setting a threshold orvoltage level which represents a duration equivalent to the maximum thatwe wish the sensor 3-10 to 'pass. This electrical effect could be, forexample, the voltage level represented by the broken line 3-174 in graphM of FIG. 3B. Assume for the discussion that the broken line 3-174 is ofconstant amplitude. If this voltage obtains on the line 3-172 theterminals of the comparator 3-150 may be chosen so that there will be anoutput at 3-164 at all times except for the condition that the signal3-126 appearing at 3-68 exceeds the level 3-174 in amplitude.

Thus, the maximum level for comparison may be set by some manuallyadjusted threshold circuit as disclosed in the first of the co-pendingapplications or it may be done automatically, as disclosed in the secondof the co-pending applications. The signal output at line 3-164 isrepresented by graph N of FIG. 3A. The elongate line 3-176 represents acontinuous signal.

Illustrated in FIG. 3A is the minimum duration memory circuit 3-70 whichis disclosed and explained in detail in the second co-pendingapplication. Briefly, the input comparator 3-84 operates into terminal3-86 of the AND gate 3-88 whose output 3-90 is applied to the shortingmeans of the integrator 3-92. A constant current supply 3-94 furnishessufficient current to the integrator to exceed leakage and assure thatthe integrator drifts upward at a very slow rate. The slope of thebroken lines 3-174 and 33-178 is exaggerated in graph M. The AND gate3-88 receives on its input terminal 3-132 the pulse 3-168 from theone-shot multivibrator 3-108 after the leading edge or ramp of the pulse3-126 has stopped increasing in amplitude. If at this time the voltageat 3-68 is'less than the voltage at 3-144, the comparator 3-84 has anoutput at 3-86 which combines with the signal at 3-132 to produce anoutput at 3-90 which energizes the discharge means of the integrator3-92 for a very short time, causing the voltage at the output 3-72 todecrease as shown at 3-180 in graph M. This output voltage at 3-72 isthe lower broken line 3-178 of the graph M. When the voltage at 3-72falls below the voltage at 3-68, the output from the comparator 3-84disappears, removing the energizing signal from the AND gate 3-88 andfrom the shorting means of the integrator 3-92, thus causing theintegrator output at 3-72 to hold at the new value which is theamplitude of the pulse 3-126. Until the next pulse, this value willremain, but slowly rising as the current supply 3-94 replenishesleakage.

The minimum duration memory circuit 3-70 will thus seek the amplitude ofthe integrated pulse 3-126 and will remember this amplitude until thenext signal comes along. It therefore seeks the amplitude correspondingto the minimum duration pulses, and enables establishment of the maximumlevel against which to compare the pulse 3-126. This is attained byslightly amplifying the output 3-72 of the integrator 3-92 in theamplifier 3-180 and providing the resulting signal 3-174 on the line3-172. The tolerance is represented by the difference between the twolevels 3-174 and 3-178, as adjusted by the manual gain control 3-162.

So long as the pulse 3-126 does not exceed the level 3-174, there willbe a signal at 3-164, the AND gate will have an output at 3-158 betweenthe times t and t the gate 3-76 will be opened for the same time, andthe output 3-78 will receive a pulse 3-184 as shown in graph R of FIG.3B which has an amplitude X, the same as the amplitude of the stretchedpulse 3-128 and of a fixed duration. The pulse 3-168 is in effect astrobing pulse which excises a portion of the stretched pulse 3-128 andpermits this portion to pass to the terminal 3-78.

The next particle pulse which is to be processed is the pulse -34 whichhas been produced by the particle Y passing on the path 26 through theaperture 22 of FIG. 1. It has two peaks 36 and 38, the amplitude of thefirst of these peaks being Y as shown. In FIG. 38, this pulse isdesignated 3-34 in graph A. Note that it has a greater duration than thepulse 3-30, and there is no assurance that the amplitude Y isproportional to the size of the particle that caused the same. It istherefore desired to disregard this pulse and not to open the gate 3-76when it is strobed by a signal equivalent to the pulse 3-168.

The pulse 3-34 crossing the low threshold 3-32 generates the rectangularwave 3-186 between the times 1,, and t as shown in graph C of FIG. 3B.This is the output of the comparator 3-28 appearing at 3-36, the inputto the leading edge detector 342. In the same manner as described inconnection with the processing of the pulse 3-30, the trigger spike3-188 (graph F) is generated, to set the RS flip-flop 3-48 and producethe beginning of the rectangular wave 3-189 (graph G). In the meantime,the pulse 3-34 is differentiated in the differentiator 3-16 so that thedifferentiated wave 3-190 (graph B) appears at the line 13-50. Since thepulse 3-34 had two peaks, there will be three points of zero slope andthese are represented at times t t and t,, where the wave 3-190 crossesthe base line. When the wave 15-190 is compared to ground 3-54 in thecomparator 3-52, only the positive portions of the wave 3-190 result inoutputs, these being the rectangular pulses 3-192 and 3-194. Thetrailing edge detector 3-62 generates the trigger spikes 3-196 and 3-198at the times t, and 1,, (graph E) which are both applied to the resetinput terminal of the RS flip-flop 3-48. The first trigger pulse 3-196serves to reset the RS flip-flop 3-48 and the second trigger spike 3-198has no effect because the flip-flop has already been reset by the spike3-196. The resetting of the RS flip-flop 3-48 ends the generation of therectangular wave 3-189 at the time I, and passing the wave 3-190 throughthe trailing edge detector 3-82 places a trigger spike 3-200 (graph H)at time I, on the line 13-98 which is the input to the second RSflip-flop 3-100.

From the above, it can be seen that the sensor responds only to thefirst peak of the wave 3-34, the signals produced by the second peakbeing ignored.

The particle pulse 3-34 is simultaneously applied to the pulse stretcher3-26 and the comparator 3-46, and

by the process described in connection with the pulse 3-30, the pulse3-34 is attenuated in the attenuator 3-40, clipped in the clipper 3-134and the resulting signal compared with the original pulse 3-34 in thecomparator 3-46. The resulting signals are shown at 3-202 in graph I ofFIG. 3B and at 3-204 in graph .1. The output from the comparator 3-46 isthe rectangular wave 3-206 of graph K whose leading edge is of noimportance but whose trailing edge precisely marks the time t that thetrailing edge of the pulse 3-34 subsided to percent of the amplitude Y.This pulse 3-206 passes through the trailing edge detector 13-102 toproduce the spike 3-208 (graph 0) which is applied by way of the line3-101 to the reset terminal of the RS flip-flop 3-100 and to the inputof the one-shot multivibrator 3-108.

The RS flip-flop 3-100 generates the durationmeasuring signal 3-210 fromthe time t,, which is the instant that the first peak of the particlepulse 3-34 occurs, to the time 1,, which is the half amplitude or thepulse 3-34. It is set by the trigger spike 3-200 and reset by thetrigger spike 13-208.

The duration-measuring pulse 3-210 opens the analog gate 3-20 and theintegrator 3-58 commences to generate the integrated pulse 3-212, theramp of which rises linearly from time t, to time t after which theplateau is generated. The integrator output 3-212 is shown in graph M,and it will be noted that the pulse 3-212 exceeds the maximum voltagelevel 3-174 from the time t, to the time t From the description of thecomparator 3-150 previously given, it can be seen that there will be nooutput from the comparator 3-150 appearing on the line 3-164 during thesaid period of time t, to r This is represented by the negativerectangular pulse 3-214 in graph N. The strobing pulse 3-216 isgenerated as shown in graph P, and the discharging pulse 3-218 isgenerated as shown in graph 0, both in the same manner as described inconnection with the processing of the pulse 3-30. Although thedischarging pulse 3-218 serves to discharge the condensers of the pulsestretcher 3-26 and the integrator 3-58, the strobing pulse 3-216 has noeffect upon the analog gate 3-76 because it occurs while the pulse 3-214occurs. This latter pulse represents an absence of signal on the line3-164, and so the eventual effect is that the pulse 3-34 does notproduce any output at the terminal 3-78. The reason for this is that theduration from its peak to its half amplitude was greater than themaximum duration represented by the voltage 3-174.

It will be noted that the processing of the pulse 3-34 had no effectupon the minimum duration memory circuit which still continues toremember the duration of the last pulse processed to produce an outputat 3-78.

Attention is now invited to the circuit of FIG. 4A and the graphs ofFIG. 4B. The illustrated sensor 4-10 is a modified form of theinvention, and it operates on the principle of differentiating theparticle pulses twice in order to find the points of minimum change ofslope of the particle pulses, these points marking the maximum slope ofthe original pulse. Since a particle pulse having more than one peak isanomalous, means are provided in the circuit for discarding any suchpulses, even if their durations are within the criterion of durationestablished by the minimum duration memory circuit.

The two particle pulses which will be considered again are substantiallythe same as those which were respectively produced by the particles Xand Y on the 13 paths 24 and 26 of FIG. 1. These'pulses are designated4-30 and 4-34 in FIG. 4B, and they are both applied to the inputterminal 4-12 of the sensor 4-10. First, the pulse 4-30 will beprocessed and such processing explained.

The pulse 4-30 is applied to the pulse stretcher 4-26 which produces thestretched pulse 4-38 of graph P of FIG. 48 having its leading edge thesame as the leading edge of the pulse 4-30 but having a flattened topwhich is at the same amplitude as the maximum amplitude of the pulse4-30, the duration of this pulse 4-38 being controlled by meanssupplying a discharge signal to the pulse-stretcher 4-26 as will beexplained hereinafter.

At the same time the particle pulse is applied to the low thresholdcircuit 4-32 which is for suppressing noise and toprovide a triggersignal to the strobing oneshot 4-108. The threshold is established bythe voltage source 4-22 and the variable resistor 4-24 providing oneinput 4-20 to the comparator 4-28, the other terminal being the line4-14 from the input terminal 4-12. The threshold for noise is shown ingraph A of FIG. 4B exaggerated at 4-44. A signal comprising arectangular wave will appear at 4-36 all the time that the inputparticle pulse 4-30 exceeds the noise threshold 4-44, this signalcomprising the pulse4-60 of graph B of FIG. 4B between the times I andThe trailing edge detector 4-42 produces a trigger spike (not shown) atthe time t, and applies this trigger spike by way of the line 4-54 tothe strobing one-shot multivibrator 4-108, the latter putting out thestrobing pulse 446 shown in the graph G between the times t, and I Thisstrobing pulse is applied'by way of the line 4-1-70 to one of the inputsto the veto-AND gate 4-130 and by way of the line 4-110 to anothertrailing edge detector 4-112. This detector produces a trigger spike(not shown) at the time i which is applied to the reset one-shotmultivibrator 4-118 by way of the line 4-116, and a reset pulse 4-48(graph H of FIG. 4B) is produced between the times t and 1 This pulse isused to discharge the pulse stretcher 4-26, to reset the integrator 4-58and to reset two toggle flip-flops which will be described. Theconnecting line for the reset functions is designated 4-120.

The incoming particle pulse 4-30 is also applied to the firstdifferentiator 4-16 which differentiates the pulse 4-30 to produce thewave 462 shown in graph C between slightly before the time t andslightly after the time t,. This signal represents the slope of theparticle pulse 4-30, and hence it has a peak at time t the point ofmaximum slope of the leading edge of the pulse 4-30, a zero-crossing atthe time I. which is the peak or zero slope condition of the pulse 4-30,and it has another peak at time t, which is the point of maximum slopeof the trailing edge of the particle pulse 4-30. This pulse is amplifiedin the amplifier 4-40 without changing its relation to time and thenapplied to a second differentiator 4-18; The output of thedifferentiator 4-18 is the differential of the wave 4-62'which appearsat 4-64 and is shown as 4-66 in graph D of FIG.- 4B. The firstdifferential of the wave 4-62 is the second differential of the wave4-30, hence the wave 4-66 represents the rate of change of the slope ofthe wave 4-30. The minimum rate of change occurs at the maximum slope onthe leading and trailing edges of the pulse 4-30, hence there will be azero crossing at the times of the peaks of the wave 4-62, namely, at thetimes and t The differentiator 4-18 operates into a low thresholdcircuit 4-74 which operates to suppress noise. The threshold 4-76 isestablished by the voltage source 4-78 and the voltage divider 4-80 and,as noted, it is below ground. The comparator 4-52 is thus constructedand arranged to produce an output only when the wave 4-66 drops belowthe noise threshold 4-76, and this occurs at the times t;, and t,,, orquite close thereto. The peak of the particle pulse 4-30 is a zerocrossing in the wave 4-62 and is a negative peak at L, in the wave 4-66.The output of the comparator 4-52 appears at 4-80 in the form of arectangular pulse shown at 4-82 in the graph E. This rectangular pulseconstitutes the duration-measuring signal of the sensor 4-10 since itmeasures the time between the points of maximum slope on the leading andtrailing edges of the particle pulse 4-30.

The rectangular pulse 4-82 is applied to the integrator 4-58 and by wayof the line 4-96 to another differentiator'4-98. In the integrator 4-58,the rectangular duration-measuring signal 4-82 is used in the mannerdescribed previously, producing a ramp and pedestal pulse 4-100 shown ingraph M of FIG. 43. From the time I to the time t the amplitude riseslinearly, integrating the rectangular pulse 4-82. From the time tonward, the integrator retains its charge 'so that the pedestal isformed, this being at an amplitude which is proportional to the durationof the pulse 4-82, and hence also proportional to the duration of thepulse 4-30 between its points of maximum slope on its leading andtrailing edges. Since the integrator 4-58 is one of the components resetby the reset one-shot 4-118, its charge subsides at the periodcommencing with the time t on account of the influence of the resetsignal 4-48.

The integrated pulse 4-100 is compared with the maximum threshold level4-102 in the comparator 4-150 and since it does not exceed this level,there is no change in the signal appearing on the line 4-164 which isone ofthe terminals of the veto-AND gate 4-130. The operation of theminimum duration memory circuit 4-70 is as explained in connection withthe equivalent circuit 3-70 of FIG. 3A except that the AND gate 4-88receives its lower input from the line 4-158, the output from theveto-AND gate 4-130. This latter signal is a rectangular wave 4-122shown in graph 0.

The rectangular duration-measuring signal 4-82 is differentiated in thedifferentiator 4-98 and puts out positive and negative trigger pulses4-102 and 4-104 (graph F) which are converted to negative trigger pulses4-114 and 4-126 (graph I) in the rectifier 4-128. These trigger pulsesare applied to the lower terminal 4-132 of the veto-AND gate 4-134 and,if passed, are applied to the first toggle flip-flop 4-136. Eachof thetrigger spikes 4-114 and 4-126 indicates a zero crossing of the wave4-66 which corresponds to maximum slope instances of the original pulse4-30.

Both of the toggle flip-flops 4-136 and 4-137 have previously been resetto zero so that no signals appear on the paths 4-138 and 4-140 betweenpulses. Under these circumstances, there is no signal, or zero logic onthe line 4-l42 and the line 4-146 leading to the veto input of the ANDgate 4-130. Likewise, there is no veto signal on the veto terminal ofthe veto-AND gate 4-134, so that trigger pulses can be received on theline 4-132. The first spike 4-114 toggles the flip-flop 4-136, producingan output in the form of a rectangular wave 4148 shown in graph J whichstarts at the first trigger pulse at time t, and ends at the secondtrigger pulse at the time r At the occurrence of the second triggerpulse, the trailing edge detector 4-152 produces a trigger spike (notshown) which triggers the toggle flip-flop 4-137 so that there is anoutput from it on the line 4-140. This consists of a rectangular pulse4-160 (g'raph K) that starts at the time t, and ends at the time 1,,when the toggle flip-flops are reset. Thus, there is a logic one signalon the upper input of the AND gate 4154 but no signal or a logic zero onthe lower input to this AND gate. The strobing pulse 4122 (Graph 0)commences at the time t, and ends at the time During this period oftime, there is no veto signal on the line 4146 because there were onlytwo zero crossings of the second differentiated wave 4-66, signifying asubstantially perfect pulse 4-30. As explained, there is a signal on theline 4164 as represented by the absence of veto signal at 4-166 in graphN, and there is the strobing signal 4-46 on the line 4-170. Since allconditions for passing a signal through the veto-AND gate 4130 are met,the signal 4-122 appears at the line 4-158, opening the analog gate4-176 and permitting a portion of the stretched pulse 4-38 to pass tothe terminal 4178 between the times 1 and I This output signal comprisesthe derived pulse 4-174 of graph O.

From the above, it appears that there are two criteria for obtaining anoutput, the first of which is the same as before with respect to themaximum level against which the integrated pulse is compared, and thesecond of which is an additional criterion which depends upon the numberof crossings of the second derivative of the original particle pulse430. This will be discussed in more detail below, in connection with theprocessing of a multipeak pulse such as 434 in the sensor 410. For themoment, it should be appreciated that even though the duration betweenthe points of maximum slope on the leading and trailing edges of a pulsemeet the first criterion, it is possible that the existence of more thanone peak indicates an anomalous pulse and hence such pulse should bediscarded. It is for this reason that the second criterion has beenbuilt into the sensor 4-10.

Considering now the pulse 3-34, when it is passed through the comparator4-28, it generates the rectangular wave 4-182 between the times I and Ithe trailing edge of the wave 4-182 initiating the strobing pulse 4-194between the times 2 and and the reset pulse 4-196 between the times 1and The first derivative of the pulse 434 comprises the wave 4-l84 whichhas two positive peaks at I and I a negative peak at I and three zerocrossings, at r r and The second derivative wave 4185 has four zerocrossings at I I t and at 1, As a result, the output of the comparator452 comprises two rectangular pulses 4-l86'and 4-188 shown at graph E.

The two rectangular pulses 4186 and 4-188 together comprise theduration-measuring signal which consists of two parts because the pulse434 has two peaks, there being four points of maximum slope. Thiscompound signal is integrated in the integrator 4-58 and produces thedouble pedestal pulse 4208. There is a first ramp between the times rand i corresponding to the rectangular pulse 4-186; a lower pedestal orplateau portion which remains at the maximum amplitude reached by thefirst ramp from the time I to the time I a second ramp between the timesI and 2,, corresponding to the rectangular pulse 4-188; and a secondpedestal portion between the times 2, and t The pulse 4-208 exceeds theupper limit threshold 4102 from the time t, (which is somewhere betweenthe times t and 2, to the time t and therefore, the line 4-164 will haveno signal. This is the equivalent of a veto signal 4-210 shown in graphN. Thus, regardless of what appears on the lines 4170 and 4-146, therewill be no signal at 4-158 and the gate 4176 will not open.

Assume for the moment that the duration of the signal composed of thetwo rectangular pulses 4186 and 4-188 is such that the integrated signal4208 does not exceed the threshold 4102. In such case, there are two ormore peaks to the pulse 4-34 which means that the amplitude is not atrue measure of the size of the particle causing the pulse. It isdesired not to pass this pulse through the sensor 4-10, notwithstandingthat its duration is substantially that of a desirable pulse. This isaccomplished by the circuitry in the lower right-hand corner of theblock diagram.

The two rectangular pulses 4-186 and 4-188 produce the positive andnegative trigger spikes 4190 and 4192 which are shown in graph F afterpassing through the differentiator 498. These spikes are converted intonegative spikes 4198 by the rectifier and then pass to the toggleflip-flops 4-136 and 4-137 through the veto-AND gate 4134. At firstthere is no output from the flip-flop 4-137 or 4136 so that there is nooutput on line 4-140, on the line 4138, on the line 4142 or on the line4-146. At the time I the first trigger spike arrives and toggles theflip-flop 4-136 to produce the start of the rectangular pulse 4-200 ofgraph J. The second trigger spike arrives at the time r and it completesthe pulse 4200. In this period of time there was an output or logic oneon line 4138, but no output, i.e., a logic zero on the line 4140, sothat the condition described before the arrival of the first triggerspike still obtains. As soon as the second trigger spike 4198 arrives,there is no longer a signal at the line 4-138 but the rectangular pulse4-204 commences. There is still no signal on the lines 4-142 and 4146.As soon as the third trigger spike 4198 arrives, at the time t it startsa second period or signal output from the first toggle flip-flop 4136,represented by the rectangular wave 4-202. This does not affect thetrailing edge detector 4-152 unit] the time I and so, for the period oftime from t until the time t there are signals on the lines 4138 and4-140. The AND gate 4154 produces a signal represented by the veto pulse4206 of graph L and this blocks the further receipt of trigger spikesthrough the gate 4-134 while at the same time vetoing the AND gate 4130and preventing it from passing the strobing signal 4194. Accordingly,there will be no output at the terminal 4178 because the analog gate4176 was not opened.

It should be noted that the receipt of three trigger spikes by thetoggle flip-flops is sufficient to block the advent of further triggerspikes to give the circuit the chance to veto the strobing pulse.

The sensor 4l0 will reject particle pulses which have only one peak buttoo long a duration. The manner in which this is done depends upon thecriterion established by the minimum duration memory circuit 4-70.

The invention is also directed to the method of discriminating betweendifferent types of desirable and undesirable pulses.

Variations can be made in the circuitry without departing from thespirit or scope of the invention as defined in the appended claims.

What it is desired to secure by Letters Patent of the United States is:

1. An axial trajectory sensor for use with a Coulter type particleanalyzing apparatus in which particles passing through an apertureproduce desirable particle pulses whose amplitudes are most nearlyproportional to their respective sizes when passing closest to an axialtrajectory through said aperture and having thereby a certainapproximate duration, and in which particles passing through saidaperture on trajectories displaced from the axis will produce otherparticle pulses whose amplitudes are not necessarily proportional totheir respective sizes and whose durations tend to be greater than saidcertain approximate duration; said sensor acting to respond to saiddesirable particle pulses and not to respond to said other pulses andcomprising:

A. input terminal means and output terminal means having a channel forpassage of electrical signals between the terminal means with gate meansin said channel to control the signals which appear-at the outputterminal means, the input terminal means adapted to have said desirableand other particle pulses applied thereto,

B. means for measuring the duration of a portion of a particle pulseapplied to said input terminal means and deriving a duration-measuringpulse of constant amplitude and having the measured duration,

i. said portion comprising the extent of particle pulse between one ofthe first-occuring maximum and minimum slopes on the leading edge of thepulse and ii. extending to a predetermined part of the trailing edge ofthe pulse,

C. means for converting said duration measuring pulse into an electricalquantity whose value is proportional to the time duration of saiddurationmeasuring pulse,

D. means for establishing an electrical effect of a standard equivalentto a maximum desired duration distinguishing between quantities producedby desirable and other pulses,

E. means comparing said quantity with said standard and providing afirst type of energizing signal if the quantity does not exceed thestandard and a second type of signal if the quantity does exceed thestandard, and

F. means for applying one of said energizing signals to the gate meansin said channel to permit passage to said output terminal means of onlyelectrical signals derived from desirable particle pulses.

2. The axial trajectory sensor as claimed in claim 1 in which means areprovided to close said gate means irrespective of the application of anenergizing signal to open said gate means in the event that a particlepulse has two points of zero slope.

3. The axial trajectory sensor as claimed in claim 1 in which theportion comprising the extent of the particle pulse commences at thefirst-occurring minimum slope of the leading edge and said measuringmeans include means for locating said first-occurring minimum slope. I

4. The axial trajectory as claimed in claim 3 in which said locatingmeans comprise a differentiator for differentiating said particle pulseonce. i I

5. The axial trajectory sensor as claimed in claim 3 in which thepredetermined part of the trailing edge at which said portion terminatesis the point at which said trailing edge subsides to a predeterminedfraction of the amplitude of the particle pulse and said measuring meansinclude means for locating said point.

6. The axial trajectory sensor as claimed in claim 5 in which saidlocating means comprise a pulse stretcher and attenuator connected toprovide a stretched attenuated signal and means for comparing theattenuated signal with said particle pulse.

7. The axial trajectory sensor as claimed in claim 6 in which the meansfor locating the first-occurring minimum slope comprise a differentiatorfor differentiating said particle pulse once.

8. The axial trajectory sensor as claimed in claim 1 in which theportion comprising the extent of the particle pulse commences at thefirst-occurring maximum slope of the leading edge and said measuringmeans include means for locating said first-occurring maximum slope.

9. The axial trajectory sensor as claimed in claim 8 in which saidlocating means comprise circuitry for differentiating said particlepulse to obtain a second differential thereof.

10. The axial trajectory sensor as claimed in claim 9 which includesmeans responsive to said differentiating circuitry to produce a train ofsignals for each particle pulse which is characteristic of the number ofpeaks of said particle pulse, and means for responding to the train ofsignals to close the gate means any time that a train of pulsessignifies the presence of a particle pulse having more than one peak.

11. The axial trajectory sensor as claimed in claim 8 in which thepredetermined part of the trailing edge at which said portion terminatesis the point of maximum slope thereof and said measuring means includemeans for locating said point.

12. The axial trajectory sensor as claimedin claim I l in which saidpoint locating means and said means for locating the first-occurringmaximum slope are the same.

13. The axial trajectory sensor as claimed in claim 11 in which saidpoint locating means and said means for locating the first-occurringmaximum slope are the same and comprise circuitry for differentiatingsaid particle pulse to obtain a second differential thereof.

14. An axial trajectory sensor for use with a Coulter type particleanalyzing apparatus in which particles passing through an apertureproduce desirable particle pulses whose amplitudes are most nearlyproportional to their respective sizes when passing closest to an axialtrajectory through said aperture and having thereby a certainapproximate duration, and in which particles passing through saidaperture on trajectories displaced from the axis will produce otherparticle pulses whose amplitudes are not necessarily proportional totheir respective sizes and whose durations tend to be greater than saidcertain approximate duration; said sensor acting to respond to saiddesirable particle pulses and not to respond to said other pulses andcomprising:

A. input terminal means and output terminal means having a channel forpassage of electrical signals between the terminal means with switchmeans in said channel to control the signals which appear at the outputterminal means, the input terminal means adapted to have said desirableand other particle pulses applied thereto,

B. means for measuring the duration of a portion of a particle pulseapplied to said input terminal means and deriving a duration-measuringpulse of constant amplitude and having the measured duration,

i. said portion comprising the extent of the particle pulse between oneof the first-occurring maximum and minimum slopes on the leading edge ofthe pulse and ii. extending to a predetermined part of the trailing edgeof the pulse,

C. means for converting said duration-measuring pulse into an electricaltime signal pulse whose amplitude is proportional to the duration ofsaid duration-measuring pulse,

D. means for establishing a voltage level representative of theamplitude equivalent to the maximum duration of desirable particlepulses,

E. means comparing the amplitude of said electrical time signal withsaid voltage level and providing a first type of energizing signal ifsaid amplitude does not exceed said level and a second type ofenergizing signal if the amplitude exceeds said level,

F. gating means provided between the comparing means and the switchmeans,

G. strobing pulse producing means connected with said switch means andcoupled with said durationmeasuring means to produce a strobing pulse atsaid gating means in timed relation to a particle pulse applied to saidinput terminal means,

H. means for applying all particle pulses through said channel to saidswitch means in synchronized relation with said strobing pulse,

l. means for applying said energizing signals to said gating means, saidgating means being constructed to pass said strobing pulse to operatesaid switch means to signal-passing condition only when said first-typeof energizing signal is applied to said gating means,

whereby said switch means will be activated to pass particle pulseswhose duration is less than said maximum duration.

15. The axial trajectory sensor as claimed in claim 14 in which meansare provided to close said gating means irrespective of the applicationof an energizing signal to open said gating means in the event that aparticle pulse has two points of zero slope.

16. The axial trajectory sensor as claimed in claim 14 in which theportion comprising the extent of the particle pulse commences at thefirst-occurring minimum slope of the leading edge and said measuringmeans include means for locating said first-occurring minimum slope.

17. The axial trajectory as claimed in claim 16 in which saidlast-mentioned means comprise a differentiator for differentiating saidparticle pulse once.

18. The axial trajectory sensor as claimed in claim 16 in which thepredetermined part of the trailing edge at which said portion terminatesis the point at which said trailing edge subsides to a predeterminedfraction of the amplitude of the particle pulse and said measuring meansinclude means for locating said point.

19. The axial trajectory sensor as claimed in claim 18 in which saidlocating means comprise a pulse stretcher and attenuator connected toprovide a stretched attenuated signal and means for comparing theattenuated signal with said particle pulse.

20. The axial trajectory sensor as claimed in claim 19 in which themeans for locating the first-occurring minimum slope comprise adifferentiator for differentiating said particle pulse once.

21. The axial trajectory sensor as claimed in claim 14 in which theportion comprising the extent of the particle pulse commences at thefirst-occurring maximum slope of the leading edge and said measuringmeans include means for locating said first-occurring maximum slope.

22. The axial trajectory sensor as claimed in claim 21 in which saidlocating means comprise circuitry for differentiating said particlepulse to obtain a second differential thereof. I

23. The axial trajectory sensor as claimed in claim 22 which includesmeans responsive to said differentiating circuitry to produce a train ofsignals for each particle pulse which is characteristic of the number ofpeaks of said particle pulse, and means for responding to the train ofsignals to close the gate means any time that a train of pulsessignifies the presence of a particle pulse having more than one peak.

24. The axial trajectory sensor as claimed in claim 21 in which thepredetermined part of the trailing edge at which said portion terminatesis the point of maximum slope thereof and said measuring means includemeans for locating said point.

25. The axial trajectory sensor as claimed in claim 24 in which saidpoint locating means and said means for locating the first-occurringmaximum slope are the same.

26. The axial trajectory sensor as claimed in claim 24 in which saidpoint locating means and said means for locating the first-occurringmaximum slope are the same and comprise circuitry for differentiatingsaid particle pulse to obtain a second differential thereof.

27. A sensor for use with a Coulter type particle analyzing apparatus inwhich particles passing through an aperture produce desirable pulseseach having a single peak and whose amplitudes are most nearlyproportional to their respective sizes and undesirable pulses eachhaving multiple peaks and whose amplitudes are not necessarilyproportional to their respective sizes; said sensor acting to respond tosaid desirable particle pulses and not to respond to the other pulsesand comprising:

A. lnput terminal means and output terminal means having a channel forpassage of electrical signals between the terminal means with switchmeans in the channel to control the signals which appear at the outputterminal means, the input terminal means adapted to have the desirableand other particle pulses applied thereto,

B. gating means for operating said switch means,

C. means connected with said input terminal means for counting thenumber of peaks in any incoming particle pulse and producing a firsttype of energizing signal if there is one peak and a second type ofsignal if there is more than one peak in said pulse, and

-D. a connection from said last means to said gatng means for applyingsaid energizing signals thereto,

the gating means being responsive to said first type of energizingsignal to operate said switch means to pass the pulse to said outputterminal means and responsive to said second type of energizing signalto render said switch means inoperative. I i

28. The sensor as claimed in claim 27 in which said counting meansinclude a circuit for obtaining a second differential of said incomingpulse, a circuit for counting the number of zero crossings of the baseline of said second differential and producing a train of pulsescharacteristic of said number, and a counter responsive to said train ofpulses and producing said'first type of energizing signal only when saidtrain of pulses comprises two pulses, and said second type of energizingsignal when said train contains more than two pulses.

29. The method of sensing between the particle pulses caused byparticles passing through a Coulter particle apparatus aperture on axisof the aperture and off the axis of the aperture, which comprises:

A. measuring the duration of that portion of the particle pulse whichfalls between one of the minimum and maximum slopes of the leading edgeand a predetermined point on the trailing edge of the pulse and derivingtherefrom a duration-measuring pulse of constant amplitude and havingsaid duration,

B. converting the duration-measuring pulse into a signal whose amplitudeis proportional to said duration,

C. establishing a signal level which represents the maximum duration ofa duration-measuring pulse for desirable pulses,

D. comparing the signal with the signal level and obtaining anenergizing signal of a first type if the signal level is not exceededand a second type if the signal level is exceeded,

E. deriving an electrical signal from each said particle pulse, and

F. passing or blocking said derived signals on the basis of whether theyhave respectively produced energizing signals of the first or secondtype.

30. The method as claimed in claim 29 in which the measuring stepincludes measuring the duration from the minimum slope of the leadingedge.

31. The method as claimed in claim 29 in which the measuring stepincludes measuring the duration from the maximum slope of the leadingedge.

32. The method as claimed in claim 29 in which the measuring stepincludes measuring the duration from the minimum slope of the leadingedge to a fractional amplitude on the trailing edge.

33. The method as claimed in claim 29 in which the measuring stepincludes measuring the duration from the maximum slope of the leadingedge to the maximum slope of the trailing edge.

34. The method as claimed in claim 29 in which the measuring stepincludes deriving the first differential of the pulse so that thezero-crossing thereof marks the point of minimum slope of the leadingedge.

35. The method as claimed in claim 33 in which the measuring stepincludes deriving the second differential of the pulse so that the firstand last zero-crossings identify the points of maximum slope on theleading and trailing edges of said pulse.

36. An axial trajectory sensor for use with a particle study apparatusin which particles pass through a detecting zone having an axis forproducing particle pulses, the particles, when passing closest to anaxial trajectory through the detecting zone, producing desirableparticle pulses having amplitudes which are most nearly proportional tothe respective sizes of the particles, and also thereby having a certainapproximate duration, and in which particles passing through thedetecting zone on trajectories displaced from its axis will produceother particle pulses having amplitudes which are not necessarilyproportional to their respective sizes and having durations which tendto be longer than said certain approximate duration; said sensor beingconstructed with an input and arranged to respond to said desirableparticles pulses in a first manner and to respond to said otherparticles pulses in a second manner and comprising:

A. means for measuring the duration of a portion of a particle pulseapplied to the sensor input and for deriving a duration-measuring pulsehaving the measured duration, said pulse portion comprising the extentof the particle pulse lying between one of the first-occurring maximumand minimum slopes on the leading edge of the pulse and extending to apredetermined part of the trailing edge of the pulse;

B. means for establishing an electrical standard equivalent to a maximumdesired duration for the duration-measuring pulses;

C. means for comparing each said durationmeasuring pulse with saidstandard and for providing a first type of energizing signal if saidstandard is not exceeded and a second type of signal if said standard isexceeded, and D. means coupled to receive said first and second types ofsignals for generating, respectively, the

first and second manners of response.

UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No. Dated ny- 1 Inventor(s) Walter R. Hogg et al It is certified that error appears.in the above-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 7, line 42, chang "2-50" to -3-50' Column 11,- line 59, change"3-l90 to 3-189 Column 16, line 49, change "unitl" to until Column 19,line 62, change "last-mentioned" .to Locating I Signed and sealed this3rd day 'o-ffDecember 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer 9 Commissioner ofPatents USCOMM-DC 60376 P69 FORM P0-1050 (10-69) U3. GOV IINH INTHUNIING OFHCE: "(I p-ll-JSI.

1. An axial trajectory sensor for use with a Coulter type particleanalyzing apparatus in which particles passing through an apertureproduce desirable particle pulses whose amplitudes are most nearlyproportional to their respective sizes when passing closest to an axialtrajectory through said aperture and having thereby a certainapproximate duration, and in which particles passing through saidaperture on trajectories displaced from the axis will produce otherparticle pulses whose amplitudes are not necessarily proportional totheir respective sizes and whose durations tend to be greater than saidcertain approximate duration; said sensor acting to respond to saiddesirable particle pulses and not to respond to said other pulses andcomprising: A. input terminal means and output terminal means having achannel for passage of electrical signals between the terminal meanswith gate means in said channel to control the signals which appear atthe output terminal means, the input terminal means adapted to have saiddesirable and other particle pulses applied thereto, B. means formeasuring the duration of a portion of a particle pulse applied to saidinput terminal means and deriving a duration-measuring pulse of constantamplitude and having the measured duration, i. said portion comprisingthe extent of particle pulse between one of the first-occuring maximumand minimum slopes on the leading edge of the pulse and ii. extending toa predetermined part of the trailing edge of the pulse, C. means forconverting said duration measuring pulse into an electrical quantitywhose value is proportional to the time duration of saidduration-measuring pulse, D. means for establishing an electrical effectof a standard equivalent to a maximum desired duration distinguishingbetween quantities produced by desirable and other pulses, E. meanscomparing said quantity with said standard and providing a first type ofenergizing signal if the quantity does not exceed the standard and asecond type of signal if the quantity does exceed the standard, and F.means for applying one of said energizing signals to the gate means insaid channel to permit passage to said output terminal means of onlyelectrical signals derived from desirable particle pulses.
 2. The axialtrajectory sensor as claimed in claim 1 in which means are provided toclose said gate means irrespective of the application of an energizingsignal to open said gatE means in the event that a particle pulse hastwo points of zero slope.
 3. The axial trajectory sensor as claimed inclaim 1 in which the portion comprising the extent of the particle pulsecommences at the first-occurring minimum slope of the leading edge andsaid measuring means include means for locating said first-occurringminimum slope.
 4. The axial trajectory as claimed in claim 3 in whichsaid locating means comprise a differentiator for differentiating saidparticle pulse once.
 5. The axial trajectory sensor as claimed in claim3 in which the predetermined part of the trailing edge at which saidportion terminates is the point at which said trailing edge subsides toa predetermined fraction of the amplitude of the particle pulse and saidmeasuring means include means for locating said point.
 6. The axialtrajectory sensor as claimed in claim 5 in which said locating meanscomprise a pulse stretcher and attenuator connected to provide astretched attenuated signal and means for comparing the attenuatedsignal with said particle pulse.
 7. The axial trajectory sensor asclaimed in claim 6 in which the means for locating the first-occurringminimum slope comprise a differentiator for differentiating saidparticle pulse once.
 8. The axial trajectory sensor as claimed in claim1 in which the portion comprising the extent of the particle pulsecommences at the first-occurring maximum slope of the leading edge andsaid measuring means include means for locating said first-occurringmaximum slope.
 9. The axial trajectory sensor as claimed in claim 8 inwhich said locating means comprise circuitry for differentiating saidparticle pulse to obtain a second differential thereof.
 10. The axialtrajectory sensor as claimed in claim 9 which includes means responsiveto said differentiating circuitry to produce a train of signals for eachparticle pulse which is characteristic of the number of peaks of saidparticle pulse, and means for responding to the train of signals toclose the gate means any time that a train of pulses signifies thepresence of a particle pulse having more than one peak.
 11. The axialtrajectory sensor as claimed in claim 8 in which the predetermined partof the trailing edge at which said portion terminates is the point ofmaximum slope thereof and said measuring means include means forlocating said point.
 12. The axial trajectory sensor as claimed in claim11 in which said point locating means and said means for locating thefirst-occurring maximum slope are the same.
 13. The axial trajectorysensor as claimed in claim 11 in which said point locating means andsaid means for locating the first-occurring maximum slope are the sameand comprise circuitry for differentiating said particle pulse to obtaina second differential thereof.
 14. An axial trajectory sensor for usewith a Coulter type particle analyzing apparatus in which particlespassing through an aperture produce desirable particle pulses whoseamplitudes are most nearly proportional to their respective sizes whenpassing closest to an axial trajectory through said aperture and havingthereby a certain approximate duration, and in which particles passingthrough said aperture on trajectories displaced from the axis willproduce other particle pulses whose amplitudes are not necessarilyproportional to their respective sizes and whose durations tend to begreater than said certain approximate duration; said sensor acting torespond to said desirable particle pulses and not to respond to saidother pulses and comprising: A. input terminal means and output terminalmeans having a channel for passage of electrical signals between theterminal means with switch means in said channel to control the signalswhich appear at the output terminal means, the input terminal meansadapted to have said desirable and other particle pulses appliedthereto, B. means for measuring the duration of a portion of a particlepulse applied to said input terminal means and deriving aduration-measuring pulse of constant amplitude and having the measuredduration, i. said portion comprising the extent of the particle pulsebetween one of the first-occurring maximum and minimum slopes on theleading edge of the pulse and ii. extending to a predetermined part ofthe trailing edge of the pulse, C. means for converting saidduration-measuring pulse into an electrical time signal pulse whoseamplitude is proportional to the duration of said duration-measuringpulse, D. means for establishing a voltage level representative of theamplitude equivalent to the maximum duration of desirable particlepulses, E. means comparing the amplitude of said electrical time signalwith said voltage level and providing a first type of energizing signalif said amplitude does not exceed said level and a second type ofenergizing signal if the amplitude exceeds said level, F. gating meansprovided between the comparing means and the switch means, G. strobingpulse producing means connected with said switch means and coupled withsaid duration-measuring means to produce a strobing pulse at said gatingmeans in timed relation to a particle pulse applied to said inputterminal means, H. means for applying all particle pulses through saidchannel to said switch means in synchronized relation with said strobingpulse, I. means for applying said energizing signals to said gatingmeans, said gating means being constructed to pass said strobing pulseto operate said switch means to signal-passing condition only when saidfirst type of energizing signal is applied to said gating means, wherebysaid switch means will be activated to pass particle pulses whoseduration is less than said maximum duration.
 15. The axial trajectorysensor as claimed in claim 14 in which means are provided to close saidgating means irrespective of the application of an energizing signal toopen said gating means in the event that a particle pulse has two pointsof zero slope.
 16. The axial trajectory sensor as claimed in claim 14 inwhich the portion comprising the extent of the particle pulse commencesat the first-occurring minimum slope of the leading edge and saidmeasuring means include means for locating said first-occurring minimumslope.
 17. The axial trajectory as claimed in claim 16 in which saidlast-mentioned means comprise a differentiator for differentiating saidparticle pulse once.
 18. The axial trajectory sensor as claimed in claim16 in which the predetermined part of the trailing edge at which saidportion terminates is the point at which said trailing edge subsides toa predetermined fraction of the amplitude of the particle pulse and saidmeasuring means include means for locating said point.
 19. The axialtrajectory sensor as claimed in claim 18 in which said locating meanscomprise a pulse stretcher and attenuator connected to provide astretched attenuated signal and means for comparing the attenuatedsignal with said particle pulse.
 20. The axial trajectory sensor asclaimed in claim 19 in which the means for locating the first-occurringminimum slope comprise a differentiator for differentiating saidparticle pulse once.
 21. The axial trajectory sensor as claimed in claim14 in which the portion comprising the extent of the particle pulsecommences at the first-occurring maximum slope of the leading edge andsaid measuring means include means for locating said first-occurringmaximum slope.
 22. The axial trajectory sensor as claimed in claim 21 inwhich said locating means comprise circuitry for differentiating saidparticle pulse to obtain a second differential thereof.
 23. The axialtrajectory sensor as claimed in claim 22 which includes means responsiveto said differentiating circuitry to produce a train of signals for eachparticle pulse which is characteristic of the number of peaks of saidparticle pulse, and means for responding to the train of signalS toclose the gate means any time that a train of pulses signifies thepresence of a particle pulse having more than one peak.
 24. The axialtrajectory sensor as claimed in claim 21 in which the predetermined partof the trailing edge at which said portion terminates is the point ofmaximum slope thereof and said measuring means include means forlocating said point.
 25. The axial trajectory sensor as claimed in claim24 in which said point locating means and said means for locating thefirst-occurring maximum slope are the same.
 26. The axial trajectorysensor as claimed in claim 24 in which said point locating means andsaid means for locating the first-occurring maximum slope are the sameand comprise circuitry for differentiating said particle pulse to obtaina second differential thereof.
 27. A sensor for use with a Coulter typeparticle analyzing apparatus in which particles passing through anaperture produce desirable pulses each having a single peak and whoseamplitudes are most nearly proportional to their respective sizes andundesirable pulses each having multiple peaks and whose amplitudes arenot necessarily proportional to their respective sizes; said sensoracting to respond to said desirable particle pulses and not to respondto the other pulses and comprising: A. Input terminal means and outputterminal means having a channel for passage of electrical signalsbetween the terminal means with switch means in the channel to controlthe signals which appear at the output terminal means, the inputterminal means adapted to have the desirable and other particle pulsesapplied thereto, B. gating means for operating said switch means, C.means connected with said input terminal means for counting the numberof peaks in any incoming particle pulse and producing a first type ofenergizing signal if there is one peak and a second type of signal ifthere is more than one peak in said pulse, and D. a connection from saidlast means to said gatng means for applying said energizing signalsthereto, the gating means being responsive to said first type ofenergizing signal to operate said switch means to pass the pulse to saidoutput terminal means and responsive to said second type of energizingsignal to render said switch means inoperative.
 28. The sensor asclaimed in claim 27 in which said counting means include a circuit forobtaining a second differential of said incoming pulse, a circuit forcounting the number of zero crossings of the base line of said seconddifferential and producing a train of pulses characteristic of saidnumber, and a counter responsive to said train of pulses and producingsaid first type of energizing signal only when said train of pulsescomprises two pulses, and said second type of energizing signal whensaid train contains more than two pulses.
 29. The method of sensingbetween the particle pulses caused by particles passing through aCoulter particle apparatus aperture on axis of the aperture and off theaxis of the aperture, which comprises: A. measuring the duration of thatportion of the particle pulse which falls between one of the minimum andmaximum slopes of the leading edge and a predetermined point on thetrailing edge of the pulse and deriving therefrom a duration-measuringpulse of constant amplitude and having said duration, B. converting theduration-measuring pulse into a signal whose amplitude is proportionalto said duration, C. establishing a signal level which represents themaximum duration of a duration-measuring pulse for desirable pulses, D.comparing the signal with the signal level and obtaining an energizingsignal of a first type if the signal level is not exceeded and a secondtype if the signal level is exceeded, E. deriving an electrical signalfrom each said particle pulse, and F. passing or blocking said derivedsignals on the basis of whether they have respectively producedenergizing signals of the first or sEcond type.
 30. The method asclaimed in claim 29 in which the measuring step includes measuring theduration from the minimum slope of the leading edge.
 31. The method asclaimed in claim 29 in which the measuring step includes measuring theduration from the maximum slope of the leading edge.
 32. The method asclaimed in claim 29 in which the measuring step includes measuring theduration from the minimum slope of the leading edge to a fractionalamplitude on the trailing edge.
 33. The method as claimed in claim 29 inwhich the measuring step includes measuring the duration from themaximum slope of the leading edge to the maximum slope of the trailingedge.
 34. The method as claimed in claim 29 in which the measuring stepincludes deriving the first differential of the pulse so that thezero-crossing thereof marks the point of minimum slope of the leadingedge.
 35. The method as claimed in claim 33 in which the measuring stepincludes deriving the second differential of the pulse so that the firstand last zero-crossings identify the points of maximum slope on theleading and trailing edges of said pulse.
 36. An axial trajectory sensorfor use with a particle study apparatus in which particles pass througha detecting zone having an axis for producing particle pulses, theparticles, when passing closest to an axial trajectory through thedetecting zone, producing desirable particle pulses having amplitudeswhich are most nearly proportional to the respective sizes of theparticles, and also thereby having a certain approximate duration, andin which particles passing through the detecting zone on trajectoriesdisplaced from its axis will produce other particle pulses havingamplitudes which are not necessarily proportional to their respectivesizes and having durations which tend to be longer than said certainapproximate duration; said sensor being constructed with an input andarranged to respond to said desirable particles pulses in a first mannerand to respond to said other particles pulses in a second manner andcomprising: A. means for measuring the duration of a portion of aparticle pulse applied to the sensor input and for deriving aduration-measuring pulse having the measured duration, said pulseportion comprising the extent of the particle pulse lying between one ofthe first-occurring maximum and minimum slopes on the leading edge ofthe pulse and extending to a predetermined part of the trailing edge ofthe pulse; B. means for establishing an electrical standard equivalentto a maximum desired duration for the duration-measuring pulses; C.means for comparing each said duration-measuring pulse with saidstandard and for providing a first type of energizing signal if saidstandard is not exceeded and a second type of signal if said standard isexceeded, and D. means coupled to receive said first and second types ofsignals for generating, respectively, the first and second manners ofresponse.